Conventional screws, often used with washers, are used to fasten sheets, panels, studs and other similar materials. An example of a typical application using a screw involves the installation of a panel roof. Such a roof typically comprises overlapping metal panels, or panels made of substances similar to metal. Of course, many applications of screws and screw/washer combinations are possible, such as the installation of panel walls or the assembly of appliances utilizing panels, an example of which is a refrigerator.
The application to a roof serves to emphasize an important function of a screw and washer combination. Of course, one major function of a screw is to fasten. However, when used with a washer for fastening roof panels, another important function of the screw is to create a seal around the aperture through which the screw passes.
If an appropriate seal is not created when the screw is driven into the panel to fasten it upon a roof, an opportunity is presented for moisture to penetrate the roof panel at the area immediately surrounding the screw. The penetration of moisture through the roof panel compromises the integrity of the roof and its ability to act as an effective moisture barrier. Moreover, the penetration of moisture around the screw may lead to corrosion around the screw. In turn, the corrosion could lead to failure of the screw as a fastener and as a seal. Thus, in the case where an adequate seal is not created by the screw with the washer, both the ability of the roof to act as a moisture barrier and its structural integrity might be adversely affected.
One of the causes of failure of a screw and washer combination, whether in a roofing or other application, is improper installation. Typically, screws are installed through operator use of a driving tool, such as an electric, manual, or pneumatic drill. The driving tool is used to drive the screw into a sheet or panel so that the lower portion of the screw head traps the washer between the screw head and the sheet or panel. The washer, in combination with the screw head and the surface of the sheet or panel, creates a seal when the washer is properly positioned and trapped between the screw head and the sheet or panel.
However, for the seal to be effective, an appropriate amount of pressure must be exerted upon the washer by the screw head. Either too little or too much pressure exerted upon the washer may reduce the ability of the washer to provide an effective seal. Insufficient pressure leaves gaps through which moisture may pass. Excessive pressure may fracture the washer by forcing it against an extrusion of the panel or sheet which is created by driving the screw into the panel or sheet. Alternately, excessive pressure may cause overcompression of the washer which causes it to spread radially beyond the fastener head, causing excessive stress and ultraviolet degradation, ultimately resulting in seal failure or leaks. Also, in some roofing systems, the same fasteners are also used to secure lapped sheet material. The proper amount of fastener pressure on these sheets helps maintain fastening integrity as well as an effective seal.
Addressing this concern, some known driving tools and installation techniques utilize torque or depth setting attachments in combination with the driving tool. The torque setting attachment allows driving of the screw until a predetermined torque value is reached. The depth setting attachment allows driving of the screw until a given depth is reached. Ideally, the predetermined amount of torque or depth results in an amount of pressure upon the washer which maximizes its ability to effectively function as a seal and prevent overdriving and premature failure due to overtorquing a fastener of a predetermined size (that is, attaining its maximum setting torque).
However, in practical terms, the use of a torque setting or similar attachment may not be efficient as a result of the installation environment. Installation typically takes place on construction sites or in manufacturing facilities, and an operator controls the driving tool used to drive the screw into the panel or sheet. The operator is frequently subject to production pressures such as completion deadlines and quotas. In many instances, the operator finds the depth or torque setting attachment to be a cumbersome device which adversely affects the ability of the operator to quickly perform the fastening task. For this reason, the operator may choose to remove the setting attachment. The failure to use a setting attachment may impair the effectiveness of the seal or the integrity of the screw itself, since too little or too much pressure may be placed on the washer by the screw, as discussed above.
Another potential defect in the seal arises from the positioning of the washer with respect to the screw and the integrity of the washer itself. Known washers include generally angular protrusions on the inner diameter surface of the washer. A function of the protrusions is to allow the washer to be easily attached to the screw. The protrusions also hold the washer in place, for example, attached to the screw prior to installation of the screw.
Another function of these protrusions is to maintain the generally centered position of the washer around the shank of the screw at a position beneath the head of the screw when the screw and washer are assembled, as well as during the driving of the screw. If the screw was mounted in a final position in an off-center position, the seal on one side of the screw may be adversely affected.
Such off-centering problems are frequently caused by the angular nature of the protrusions on the inner diameter surface of the washer. The edges of the angular protrusions have a tendency to catch the screw and threads and tear during mounting of the washer to the screw and during driving of the screw into a panel or sheet. If one of the protrusions is torn, the washer may not remain centered during driving of the screws. The catching and tearing may also lead to tearing of the entire washer which prevents the washer from functioning properly.
Thus, there is a need for a screw for use with a washer which acts as an effective fastener and seal, even when a depth or torque setting attachment is not used in combination with a driving tool by an operator. Additionally, there is a need for a screw with a washer which is easily attached and held on the screw, and which reliably maintains its position during driving of the screw while resisting catching and tearing.